Process for the preparation of powdery homo-or copolymers of ethylene

ABSTRACT

A process for the recovery of powdery homopolymers or copolymers of ethylene from a polymerization system is disclosed, wherein the corresponding monomers are subjected to polymerization conditions in a reactor and the resulting polymer, together with unconverted monomer, is discharged from the reactor and expanded to a lower pressure. At least part of this mixture is quenched by the addition of a cold gas, which gas is at least one of the polymerization monomers, to a temperature below the melting point of the polymer to solidify the polymer contained in such mixture as the form of a fine powder. Thereafter the powder is separated from the mixture and remaining unconverted monomer is recycled to the reactor. The process involves simpler equipment and reduced utility costs as compared to prior processes.

United States Patent [191 Frielink 1 March 6, 1973 [751 Inventor:Johannes M.

Netherlands [73] Assignee: Stamlcarbon N. V.,

Netherlands [22] Filed: June 23, 1971 [21] Appl. No.: 155,798

Frielink, Sittard,

Heerlen [30] Foreign Application Priority Data June 29, 1970 Netherlands..7009555 [56] References Cited UNlTED STATES PATENTS Ehrlich et al...260/94.9 GD

6/ 1966 Eilbracht et a1. ..260/94.9 R 3/1968 Turner et a1 ..260/88.1 R

Primary Examiner-Harry Wong, Jr. Attorney-Cushman, Darby & Cushman [57]ABSTRACT A process for the recovery of powdery homopolymers orcopolymers of ethylene from a polymerization system is disclosed,wherein the corresponding monomers are subjected to polymerizationconditions in a reactor and the resulting polymer, together withunconverted monomer, is discharged from the reactor and expanded to alower pressure. At least part of this mixture is quenched by theaddition of a cold gas, which gas is at least one of the polymerizationmonomers, to a temperature below the melting point of the polymer tosolidify the polymer contained in such mixture as the form of a finepowder. Thereafter the powder is separated from the mixture andremaining unconverted monomer is recycled to the reactor. The processinvolves simpler equipment and reduced utility costs as compared toprior processes.

1 1 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION Homopolymersand copolymers of ethylene are prepared by subjecting ethylene, mixed,in the case of copolymer production, with one or more unsaturated,organic compounds copolymerizable therewith, to elevated temperaturesand pressures, e.g., l20- 400C and 500 5,000 atmospheres, in a reactorin the presence of a polymerization initiator. This high-pressureprocess is conducted continuously by continuously discharging from thereactor a mixture which contains the resulting polymer and a quantity ofunconverted monomer, recovering the polymer after expansion of themixture and then returning to the reactor, after compression to thereactor pressure, the remaining gaseous monomer, together with aquantity of fresh monomer corresponding to the amount of monomer whichhas been converted into polymer.

It is desired to use high reaction temperatures in the reactor becauseof the consequential advantage in high ethylene conversion andpolymerization rates. At this high reaction temperature, the temperatureof the mixture discharged from the reactor will, even after expansion,still be above the melting point of the polymer, so that the polymer isremoved from the mixture in the liquid state. The liquid polymer soremoved may be transported to a device wherein it is given the form (forinstance, granules or powder) desired by the end user. As a rule such adevice consists of an extruder which forms the polymer into the form ofstrands which are thereafter cooled in a water bath to below the meltingpoint of the polymer and are thereafter reduced to granules by means ofa granulating device. If a powdery product is required, for instance,for applications involving rotational moulding, whirl-sintering orspraycoating, the granules are ground to the desired fineness.

Because of the high viscosity of the polymer recovered in the liquidstate, difficulties arise during the separation and transportation ofsuch polymer. For this reason, it is difficult to prepare polymers witha low melt index, such as, for instance, polymers used for themanufacture of tubing, wherein a polymer melt index of less than 0.2 (asmeasured according to ASTM D 1238-62 T) is required. If a powderyproduct is to be prepared, the use of extruders and grinders isrelatively complicated and clumsy.

The prior art has proposed to expand the mixture withdrawn from thepolymerization reactor in such a way that the liquid polymer is presentin the gas flow as a mist of fine droplets. Subsequently this mist isquenched to a temperature below the melting point of the polymer bywater injection, to produce an aqueous suspension of polymer powder. Thepowder is separated off, e.g., by filtration, from the suspension andsubsequently dried. The unconverted monomer from which the polymer hasbeen separated is saturated with water. vapor, which must be removedbefore such monomer is returned to the reactor. This process requiresrather complex apparatus because of the extra steps which are necessaryfor the aftertreatment of the polymer and the drying of the unconvertedmonomer (see U.S. Pat. No. 2,831,845).

It is also known to cool the mixture leaving the polymerization reactorby external cooling to a temperature slightly above the melting point ofthe polymer and thereafter to expand such cooled mixture to produce asolid polymer powder. Such a process is most difficult to controlbecause the temperature of the mixture leaving the polymerizationreactor is not constant, but may be subject to wide fluctuations, andsince the ultimate cooling at which the polymer changes into the solidstate occurs, in this process, during the expansion step, there is adecided risk that the exchange device will be clogged by solid particlesand that the solidified particles will agglomerate (see U.S. Pat. No.3,090,778).

DESCRIPTION OF THE INVENTION The present invention provides animprovement on the prior art processes acknowledged above. The presentinvention relates, as mentioned previously, to a process wherein powderyhomo polymers or copolymers of ethylene are prepared by subjectingmonomer, which monomer is ethylene mixed with 0 50 percent of at leastone other unsaturated organic compound copolymerizable with ethylene, ina reactor to elevated temperatures and pressures in the presence of apolymerization initiator. The polymer formed in the polymerizationreactor is discharged from the reactor, together with a quantity ofunconverted monomer. This mixture is expanded to a lower pressure and atleast a part of such expanded mixture is quenched to solidify thepolymer in the form of a fine powder. The powder is separated from theremaining mixture and the remaining unconverted monomer is recycled tothe reactor after being compressed to substantially the reactorpressure. The quenching is effected by the addition of a quantity ofcold gas, which gas consists essentially of at least one of the monomersto be polymerized, to the expanded mixture. Thereafter, the polymer isremoved from the quenched mixture, preferably by means of a cyclone.Thus, a dry powdery polymer is obtained which does not require furtherprocessing in order to be suitable for use in the manufacture ofarticles by sintering or.similar process steps, and which polymer,irrespective of the melt index thereof, can be transported and storedwithout difficulty. At the same time a gaseous monomer or monomermixture is obtained which can be returned directly to the reactor aftercompression without having to be subjected to special purificationsteps. The apparatus used to practice the present invention is thereforeconsiderably simpler than that used in the known process acknowledgedabove, as a result of the elimination of devices for polymeraftertreatment and for the drying of unconverted monomer.

The temperature of the reaction mixture after the quenching step isdetermined by the temperature at which the mixture is discharged fromthe reactor, the degree of expansion to which the mixture is subjected,and the temperature and quantity of the gaseous quenching monomer addedto the mixture. As the quantity of converted monomer (polymer) in thismixture generally amounts to about 20 percent, if the quenching of themixture discharged from the reactor is exclusively by addition of themake-up monomer supplied to the reactor in replacement of monomerpolymerized, into polymer, it will be necessary for this quenchingmakesup monomer to be cooled to a very low temperature in order toquench the reaction mixture to a temperature below the melting point ofthe polymer. However, the present invention also contemplates reducingthe temperature of the mixture discharged from the reactor by externalcooling before or during the expansion to a point such that the mixtureafter expansion and before quenching is just above the melting point ofthe polymer, and in such case a smaller quantity of heat must bewithdrawn from the mixture after the expansion to solidify the polymertherein, so that the temperature of the quenching make-up monomer may beproportionally higher.

The solidification of the polymer particles, according to the presentinvention, only occurs after the mixture discharged from the reactor hasbeen expanded, with the ultimate cooling to solidify the polymereffected by the addition of the cold monomer, and therefore there is norisk of the expansion device becoming clogged by solidified polymer,which is a distinct drawback to the prior art process acknowledgedabove.

It is also possible, according to the present invention, to introduce anexcess quantity of cold monomer into the mixture discharged from thereactor after expansion thereof. In this case, wherein the quantity ofcold monomer added is more than the amount necessary to replace themonomer which has been converted into polymer, the gaseous monomerremaining after polymer removal may be separated into a portion which isreturned to the reactor and a portion which, after the incorporation ofa quantity of monomer corresponding to the amount of monomer convertedinto polymer, is cooled and reintroduced into the mixture leaving thereactor as the quenching agent. In this manner, the quantity ofquenching agent is increased so that the temperature of this quenchingmonomer may be higher than would be the case if only the make-up monomerto be supplied to the reactor serves as the quenching agent.

The adiabatic expansion of the mixture discharged from. the reactorproduces the largest cooling result when the expansion is to atmosphericor to sub-atmospheric pressure. In this case, however, the unconvertedmonomer must be compressed to a greater degree to be at the reactorpressure and therefore, in view of the cost of compression energy, it isgenerally advantageous not to expand the mixture completely toatmospheric pressure or below. In practice, this means that a compromisewill be chosen between the final expansion pressure and the quenchingmonomer pressure which is most favorable from an energy viewpoint. Theembodiments mentioned above, e.g., external cooling of the mixtureleaving the reactor prior to or during expansion, and application of anexcess quantity of quenching agent, can be used not only to allow theuse of a higher quenching agent temperature, but also to allow theutilization of a higher pressure of the mixture after expansion.However, in any event, a certain expansion, on the order of at least 100times the volume of the mixture discharged from the reactor is necessaryto obtain the required mist formation.

If a product in granulate form, in addition to a powdery product, isrequired, the mixture discharged from the reactor can be divided intotwo parts. Polymer may be removed in the liquid state, in the mannerused by the prior art acknowledged above, from a first portion and theother second portion may, after expansion, be mixed with cold quenchingmonomer in accordance with the present invention. In such a situation,the second portion is preferably chosen of such a size that a quantityof monomer corresponding to the amount of monomer converted into polymerwill suffice to cool this second portion to below the polymer meltingpoint. In this case, the extra compression energy required is limited tothat necessary to compress the second portion, from which the polymerhas been removed in the form of powder, from the corresponding low finalexpansion pressure to the pressure, generally about 250 atmospheres, towhich the mixture is normally expanded when the polymer is removed inthe liquid state. However, this embodiment is limited to applicationswherein polymers are produced having a melt index so high that theprocessing of the liquid polymer does not involve substantialdifficulties, and wherein it is desired for part of the production to beobtained in granular form. V

The quantity of ethylene in the monomer mixture in the polymerizationreactor will be at least 50 mole percent with the comonomer amounting to0 50 mole percent. Suitable comonomers are well known to the art andinclude, for instance, unsaturated esters, preferably lower alkylunsaturated esters, such as, for instance, ethyl acrylate, vinylacetate, or vinyl propionate, alkene compounds, preferably alkeneconipounds having from three to 10 carbon atoms, such as, for instance,propylene and neo-hexane or other compounds, such as acrylonitrile andvinyl chloride. A further description of suitable comonomers which canbe copolymerized with ethylene in the practice of the present inventionare disclosed in U.S. Pat. No. 3,372,153, the disclosure of which ishereby incorporated by reference.

The polymerization reaction is conducted in a manner known to the priorart, in the presence of at least one initiating agent. These initiatingagents are i well-known and include oxygen, peroxides, such as, forinstance, diethyl peroxide, di-tertiary butyl peroxide,

lauroyl peroxide, capryloyl peroxide, tertiary butyl per-' benzoate, aswell as azo compounds, azines and oximes. Mixtures of peroxides may beused with such mixtures preferably being chosen such that the differentperoxides have different lives of exchange. Instead of these freeradical initiators, coordination catalysts commonly used for the lowpressure polymerization of alpha olefins, and generally referred to asZieglar-type catalyst, may be used. Such coordination catalysts may be,for instance, titanium tetrachloride or titanium trichloride incombination with an aluminum organo compound, such as aluminum trialkylsor aluminum alkyl halides. A further description of these initiatorswillbe found in U.S. Pat. Nos. 3,293,233 and 3,373,148, the disclosure ofwhich are hereby incorporated by reference. The initiator may besupplied to the polymerization reactor as a suspension in a diluent orin theform of a solution. The quantity of initiator supplied to thereactor is quite small and is generally, for instance, in the range of0.001 3 mole percent, based on the total quantity of material introducedinto the reactor.

Chain transfer agents may also be introduced into the reactor, as iswell-known to the art. Such chain transfer agents include hydrogen,saturated hydrocarbons, generally lower alkanes, such as, for instance,propane, butane, isobutane, pentane, in an amount corresponding to about0.5 to about mole percent, based on the moles of ethylene. Furthermore,other known additives, such as anti-oxidants, slip agents, anti-staticagents, and the like may also be utilized.

Ethylene polymerization or copolymerization generally takes place at atemperature of about 120C to about 400C, depending upon the nature ofthe initiator used. During polymerization, a considerable quantity ofheat is liberated (about 800 kcal/kg of polyethylene) which must berapidly withdrawn in order to prevent the temperature from rising to thepoint where ethylene explosively decomposes into carbon, hydrogen andmethane. Normally, the conversion of ethylene into polyethylene orethylene copolymers amounts to about 12 to about 20 percent.

When the ethylene polymer is to be used for rotational mouldings, theparticle size of the polymer powder is preferably about 100 to 800microns, and for whirl-sintering and textile coating applications, thepolymer powder particle size is generally about 50 to 300 microns. Theparticle sizes are generally controlled by the choice of the size of thedischarge opening in the atomizer, with the smaller opening used for thesmaller desired particle size. Generally, the atomizer discharge openingwill be in the range of 0.5 to 5 millimeters.

DESCRIPTION OF THE DRAWINGS The invention will be more readilyunderstood by reference to the accompanying drawings wherein:

FIG. 1 represents a schematic diagram of the process of the presentinvention.

FIG. 2 is a longitudinal cross-section of the atomizer and the mixinghead of FIG. 1.

FIG. 3 represents a portion of the flow diagram of FIG. 1, showing adifferent embodiment thereof.

FIG. 4 is a fiow diagram illustrating yet another embodiment of theprocess of the present invention.

In FIG. 1, ethylene is added to reactor 1 by way of line 2 at thereactor pressure, which may be, for instance, 2,000 atmospheres. Thereactor 1 may be constructed as a tubular reactor, as illustrated, or asan autoclave. The ethylene may be added to the reactor at variouslocations, as known to the art. Ifa copolymer of ethylene with anothermonomer is to be prepared, a mixture of ethylene and such comonomers isintroduced into reactor 1 through line 2, with the quantity of ethylenein such mixture amounting to at least 50 percent. A polymerizationinitiating agent for the formation of free radicals is added to reactor1 by way of line 3. The polymerization initiating agent may also beadded at various locations and, as known to the art, the initiatingagent may vary in composition depending upon the point of addition tothe reactor.

As mentioned above, a considerable quantity of heat is liberated duringpolymerization, and this heat must be rapidly withdrawn in order toprevent the explosive decomposition of ethylene. The heat is withdrawnby cooling the reactor wall by way of a coolant supplied by line 5 tocooling jacket 4, and discharged by line 6, and also by continuouslydischarging from the reactor,

along with polymer formed therein, a quantity of unconverted monomer,and replacing this discharged monomer by cold make-up monomer. Thepolymer and: unconverted monomer in the reactor are discharged throughline 7 provided with an expansion valve 8, through which the pressure ofthe mixture discharged from the reactor decreases virtually toatmospheric pressure. As will be seen more clearly in FIG. 2, themixture discharged from the reactor passes through a tube section 9,provided with an atomizer 10 at the end thereof. The strong expansion ofthe monomer and the outflow through the atomizer causes the polymerpresent in the mixture to be sprayed as a mist of fine droplets intowider line or tube 11. Mixing head 12 is located near the inlet of tube11 and is fed a quantity of cold monomer which flows through apertures13 into tube 11 and intensively mixes therein with the mixture passingthrough the atomizer. The amount of cold quenching monomer suppliedthrough apertures 13 is so chosen that the temperature of the resultingmixture in tube section 11 decreases to below the melting point of thepolymer. As a result of this temperature drop, the fine polymer dropletssolidify into a fine powder prior to having the opportunity to combineinto larger units or to deposit on the wall of the tube 11. The finenessand grain size distribution of the polymer powder are chiefly determinedby the selection of the atomizer, so that by choice of a suitableatomizer, which choice is well within the skill of those in the art, theparticle size required for the desired purpose can be obtained. Ifnecessary, simple experiments can establish the choice of the properatomizer.

The mixture of gaseous monomer and polymer powder flows through line 11into cyclone 14 wherein the powder is separated from the gaseous monomerand is discharged, by way of collecting vessel 15 through line 16. Thegaseous monomer leaves the cyclone 14 by way of line 17. Distributorvalve 18 divides the gaseous monomer into a part that flows through line19 and one or more compressors 20, wherein the gaseous monomer iscompressed to the required feed pressure in a number of stages, and thenis fed to reactor 1, and into a part which is returned to the mixinghead 12 by way of line 21, pump 22 and cooler 23. A portion of thequenching monomer fed to the cooling device is fresh monomer,corresponding to the amount of monomer converted into polymer,introduced by way of line 24. The amount of monomer circulating ascoolant therefor is larger than the amount of fresh make-up monomersupplied through line 24.

The temperature of the mixture leaving the reactor is substantiallyhigher than the melt temperature of the polymer, which melt temperatureis generally about 1 10C. In the embodiment represented by FIG. 3, themixture of unreacted monomer and polymer discharged from reactor 1through line 7 flows through a cooling device 25, where external coolingdecreases the temperature of the mixture to a point somewhat above themelting point of the polymer, preferably at least 10C above the meltingpoint of the polymer, for instance, to C. For the quenching of thismixture, less heat must be withdrawn from the system, so that a higherfinal expansion pressure may be used or a higher quenching monomertemperature can be utilized. If

desired, a smaller amount of quenching monomer may now be circulatedthrough cooler 23.

In another embodiment represented by FIG. 4, the mixture discharged fromreactor 1 through line 7 is divided by a dividing mechanism 26 into afraction which flows through line 27 and expansion valve 8 wherein suchfraction expands to a pressure which is substantially atmospheric, andinto a second fraction which flows by way of line 28 and reducing valve29 wherein the second fraction expands to a pressure of, for instance,250 atmospheres. The fraction of the mixture flowing through expansionvalve 8 is quenched in a manner similar to that described in FIG. 1 to atemperature below the melting point of the polymer by the addition ofcold quenching monomer through mixing head 12. After the polymer powderhas been separated from the gaseous monomer in cyclone 14, the monomeris fed directly to compressor 20 by way of line 17. Since only a portionof the mixture of monomer and polymer discharged from the reactor isquenched, no excess monomer need by circulated through a mixing head,and it is possible to introduce a quantity of monomer by way of line 24and cooler 23 to mixing head 12 which corresponds to the amount ofmonomer converted into polymer.

The fraction of the mixture discharged from the reactor which passesthrough reducing valve 29 flows into a separator 30, wherein polymer isseparated in the liquid state and discharged by way of line 31. Thepolymer is subsequently subjected to conventional treatment, notillustrated, such as degasification, extrusion, granulation and thelike. The remaining monomer, which has a pressure of about 250atmospheres is lead by way of line 32 directly to an intermediate stageof compressor 20.

EXAMPLES OF THE INVENTION The invention will be understood more readilyby reference to the following examples; however, these examples areintended to illustrate the invention and are not to be construed tolimit the scope of the invention.

EXAMPLE I In the apparatus described in FIG. 1 of the accompanyingdrawings, 15,600 g/h of ethylene were polymerized at a pressure of 1,900atmospheres and a temperature of 251C. 0.04 g/h of di-tertiary butylperoxide was added to the ethylene as the initiator and 2,364 g/h ofpolyethylene having a melt index of 0.002 were produced. The mixturedischarged from the reactor was expanded to atmospheric pressure throughan atomizer having a 3 mm discharge opening, which expansion reduced thetemperature of the mixture to 235C. 30,000 g/h of cold ethylene, havinga temperature of -C, were added to the mixture by way of mixing head 12,with the temperature of the resulting mixture being 75C. The productpolyethylene was separated from the resulting mixture as a fine powderhaving the following size distribution: 5 percent smaller than 50microns, 26 percent having a size from 50 to 200 microns, 37 percenthaving a size from 200 to 400 microns, 28 percent having a size from 400to 600 microns and 4 percent being courser than 600 microns,corresponding to an average particle size of 300 microns.

EXAMPLE n The experiment of Example I was repeated by polymerizing acorresponding amount of ethylene at a pressure of 1,000 atmospheres anda temperature of 250C. 0.46 g/h of di-tertiary butyl peroxide was addedto the 15,600 g/h of ethylene and 2,449 g/h of polyethylene having amelt index of 310 was produced. The temperature in the mixturedischarged from the reactor, after expansion to atmospheric pressure,was about 190C and decreased to 60C after admixture with 30,000 g/h ofethylene having a temperature of minus 10C. This high melt indexpolyethylene was obtained as a fine powdery product 90 percent of whichhaving a particle size between 50 and 600 microns, the average particlesize being 310 microns.

EXAMPLE III In the same manner as described for Example I, 14,600 g/h ofethylene were polymerized, using 0.11 g/h of tertiary butyl perbenzoateas initiator and 4.2 mole percent of propane as chain transfer agent, at1,850 atmospheres and 223C. 2,260 g/h of polyethylene having a meltindex of 0.18 were produced. After expansion to 1 atmosphere, the temperature of the mixture discharged from the reactor was about 200C, andthis dropped to about C after admixture with 30,000 g/h of monomerhaving a temperature of 0C. The polyethylene produced was obtained as afine powder 92 percent of which having a particle size ranging from 50to 600 microns, the average particle size being 280 microns.

EXAMPLE IV In the same manner as in Example 1, 15,080 g/h of ethylenewere copolymerized with 837 g/h of vinyl acetate at a pressure of 1,900atmospheres and a temperature of 190C. 0.30 g/h of capryloyl peroxidewas added as initiator and 2.2 mole percent of propane was added aschain transfer agent. 1,960 g/h of ethylene vinyl acetate copolymerhaving a vinyl acetate content of 1.8 mole percent and a melt index of0.005 was produced. After expansion to 1 atmosphere, the temperature ofthe mixture of monomers and polymer discharged from the reactor wasabout 160C, and after admixture with 20,000 g/h of a monomer mixture ofethylene and 1.8 mole percent vinyl acetate at 0C, this temperaturedropped to about 70C. As in the preceding examples, a very fine powderwas obtained percent of which having a particle size ranging from 50 to600 microns, the average particle size being 275 microns. I

EXAMPLE V The experiment of Example I was repeated, whereby thetemperature of the mixture leaving the reactor was reduced to C byexternal cooling prior to the expansion. After expansion to atmosphericpressure the 1. ln a process for the preparation of powdery homopolymersor copolymers of ethylene, wherein (a) monomer, consisting of ethyleneand 50 mole percent of at least one other unsaturated organic compoundcopolymerizable therewith, is subjected to elevated temperature andelevated pressure in a reactor in the presence of a polymerizationinitiator, (b) the resulting liquid polymer, together with unconvertedmonomer, is discharged from the reactor and expanded to a lowerpressure, (c) the expanded mixture, at least in part, is quenched by theaddition of a cold medium to solidify polymer contained in said mixtureto a fine powder, (d) thereafter the powder is separated from themixture, and (e) the remaining monomer is compressed to substantiallysaid elevated pressure and returned to the reactor, the improvementcomprising quenching said expanded mixture with a cold gas whichconsists essentially of at least one of the monomers to be polymerized.

2. The process as claimed in claim 1, wherein the mixture of polymer andunconverted monomer discharged from the reactor is expanded to at least100 times its volume.

3. The process as claimed in claim 1, wherein said elevated temperatureis about 120 to about 400C, and said elevated pressure is about 500 toabout 5,000 atmospheres.

4. The process as claimed in claim 1, wherein said other unsaturatedorganic compound is selected from the group consisting of unsaturatedesters, alkene compounds, acrylonitrile and vinyl chloride.

5. The process as claimed in claim 1, including the step of adding from0.5 to 10 mole percent, based on the amount of ethylene, of a chaintransfer agent to said reactor.

6. The process as claimed in claim 1, wherein the temperature of themixture of polymer and unconverted monomer discharged from the reactoris reduced, before or during the expansion, by external cooling to atemperature from 10 to 40C above the melting point of the polymer.

7. The process as claimed in claim 1, wherein the amount of said coldgas is at least equal to the amount of the monomer polymerized in saidreactor.

8. The process as claimed in claim 7, wherein an excess quantity of coldmonomer is introduced into the mixture of monomer and polymer dischargedfrom the reactor, and the unconverted monomer remaining after thepolymer has been removed is divided into a portion which is returned tothe reactor and a second portion which, after having been admixed with aquantity of fresh monomer corresponding to the amount of monomerconverted into polymer, is cooled and again used to quench the mixtureof polymer and monomer discharged from the reactor.

9. The process as claimed in claim 1, wherein the mixture of polymer andmonomer discharged from the reactor divided into a part which isexpanded and from which the polymer is separated in the liquid state andinto a second part which is expanded and is thereafter quenched withcold monomer, said second part being in an amount such that a quantityof monomer corresponding to the amount of monomer converted into polymerwill cause the second part to be cooled below the melting point of saidpolymer.

10. The process as claimed in claim 1, wherein the temperature of saidcold gas is at least C below the melting point of the polymer.

11. The process as claimed in claim 10, wherein the temperature of saidcold gas is to C less than the melting point of the polymer.

1. In a process for the preparation of powdery homopolymers orcopolymers of ethylene, wherein (a) monomer, consisting of ethylene and0 - 50 mole percent of at least one other unsaturated organic compoundcopolymerizable therewith, is subjected to elevated temperature andelevated pressure in a reactor in the presence of a polymerizationinitiator, (b) the resulting liquid polymer, together with unconvertedmonomer, is discharged from the reactor and expanded to a lowerpressure, (c) the expanded mixture, at least in part, is quenched by theaddition of a cold medium to solidify polymer contained in said mixtureto a fine powder, (d) thereafter the powder is separated from themixture, and (e) the remaining monomer is compressed to substantiallysaid elevated pressure and returned to the reactor, the improvementcomprising quenching said expanded mixture with a cold gas whichconsists essentially of at least one of the monomers to be polymerized.2. The process as claimed in claim 1, wherein the mixture of polymer andunconverted monomer discharged from the reactor is expanded to at least100 times its volume.
 3. The process as claimed in claim 1, wherein saidelevated temperature is about 120* to about 400*C, and said elevatedpressure is about 500 to about 5,000 atmospheres.
 4. The process asclaimed in claim 1, wherein said other unsaturated organic compound isselected from the group consisting of unsaturated esters, alkenecompounds, acrylonitrile and vinyl chloride.
 5. The process as claimedin claim 1, including the step of adding from 0.5 to 10 mole percent,based on the amount of ethylene, of a chain transfer agent to saidreactor.
 6. The process as claimed in claim 1, wherein the temperatureof the mixture of polymer and unconverted monomer discharged from thereactor is reduced, before or during the expansion, by external coolingto a temperature from 10* to 40*C above the melting point of thepolymer.
 7. The process as claimed in claim 1, wherein the amount ofsaid cold gas is at least equal to the amount of the monomer polymerizedin said reactor.
 8. The process as claimed in claim 7, wherein an excessquantity of cold monomer is introduced into the mixture of monomer andpolymer discharged from the reactor, and the unconverted monomerremaining after the polymer has been removed is divided into a portionwhich is returned to the reactor and a second portion which, afterhaving been admixed with a quantity of fresh monomer corresponding tothe amount of monomer converted into polymer, is cooled and again usedto quench the mixture of polymer and monomer discharged from thereactor.
 9. The process as claimed in claim 1, wherein the mixture ofpolymer and monomer discharged from the reactor divided into a partwhich is expanded and from which the polymer is separated in the liquidstate and into a second part which is expanded and is thereafterquenched with cold monomer, said second part being in an amount suchthat a quantity of monomer corresponding to the amount of monomerconverted into polymer will cause the second part to be cooled below themelting point of said polymer.
 10. The process as claimed in claim 1,wherein the temperature of said cold gas is at least 80*C below themelting point of the polymer.